The present invention relates in general to new and improved tone arm systems for high fidelity record players and in particular to a tone arm system capable of tracking the grooves of a warped record at very low tracking force.
Proper stylus tracking is essential to the high fidelity reproduction of recorded signals in a phonographic type of record player and requires that the stylus be in contact with the walls of the record groove at all times. Modulations in the groove, such as high velocity high frequency audio signals, lead-in and lead-out transistions, record warps and low frequency audio signals having large excursions, can generate substantial upward forces. These forces are primarily reactive, corresponding to the accelerations of the stylus tip mass at high frequencies and, at low frequencies, to the effective mass of the tone arm system which includes the tone arm as well as the cartridge normally carried by the latter.
If the sum of these forces exceeds the downward-acting tracking force, mistracking will occur, i.e. the stylus will leave its groove. Any reduction of the tracking force, e.g. for the purpose of reducing wear on the record and on the stylus, will only increase the probability of mistracking unless the stylus reactive forces caused by groove modulations can also be reduced. The problem is particularly pronounced with respect to low frequency tracking behavior, e.g. the tracking on a warped record of recorded low audio frequency signals having large excursions, due to the fact that record warps usually exhibit a predominant frequency range from 0.5 to 10.0 Hz, while the audio frequency range generally begins in the vicinity of 30 Hz.
In a conventional tone arm system low frequency tracking is governed primarily by four system parameters: Stylus compliance, stylus damping, tone arm effective mass and tone arm pivot damping. The force required to track a record groove is determined by the product of the mechanical driving point impedance and the velocity of the stylus. The tracking force required in the low frequency range is determined by the product of the aforesaid impedance and the vertical stylus velocity due to record warps and low frequency audio modulation at any given frequency.
In order to reduce the wear on the record grooves and on the stylus it is desirable to maintain the tracking force as low as possible. Various solutions have been proposed in the past in an attempt to track warped records without the necessity for applying unduly large tracking forces detrimental to the record grooves and to the stylus. One such system is disclosed in U.S. Pat. No. 3,830,505, which uses either a low pass filter to separate the cartridge signals from the audio output, or a pickup device separate from the cartridge and stylus to isolate it from the audio signals. The servo feedback signal so obtained is amplified and energizes a linear magnetic motor to apply a compensating force to the tone arm intended to keep the cartridge from changing its position relative to the surface of the record.
A similar system is disclosed in U.S. Pat. No. 3,623,734 wherein servo action is again used to maintain the pickup in a position which is at a predetermined distance from the disk surface. In the system disclosed in this patent the feedback signal is modified to improve the operating characteristics of the system, e.g. to speed up the response time and thereby maintain within bounds the deviation represented by the feedback signal.
Experience has shown that the operation of a tone arm system wherein the servo action is directed to maintaining the cartridge position constant relative to the surface of the record does not provide optimum performance, particularly in the low frequency range. In such a system, record warp still produces occasional mistracking, particularly at the desired low levels of tracking force. In order to avoid the mistracking of warped records relatively high tracking forces must be used, particularly in the low audio frequency range, with the result that wear on the record and on the stylus are increased.
Record warp is rarely if ever purely radial but is instead usually skewed to some degree. Hence the force generated on the tone arm by the record warp will usually posses lateral, i.e. horizontal, as well as vertical components although the latter tend to predominante. Such lateral forces will also cause mistracking errors which are particularly undesirable in stereo recordings because in the latter, balance is very important but such lateral forces tend to be unevenly distributed on the two walls of the record groove, and impair the desired balance.
It is a primary object of the present invention to provide a tone arm system for a high fidelity record player wherein improved performance over comparable prior art systems is obtained.
It is another object of the present invention to provide an improved tone arm system wherein high-fidelity performance is obtained even at low frequencies through the use of record warp compensation which tone arm system permits the use of low tracking forces.
It is still another object of the present invention to provide a servo-controlled tone arm system wherein record warp is compensated for by the action of the feedback loop which modifies the system parameters to obtain a selected resonance quality factor and a selected resonant frequency.
It is yet another object of the present invention to provide high-fidelity reproduction of low audio frequency signals recorded on a warped record using low tracking forces, wherein servo-controlled feedback compensation is employed to modify the system parameters and provide a predetermined resonance quality factor and a resonant frequency that falls between the upper limit of the expected range of warp frequencies and the lower limit of the audio frequency range.
The resonant frequency of a tone arm system is determined by the effective mass of the system and by the compliance of the stylus. The resonance quality factor Q of the system is determined by the damping at the stylus and at the tone arm pivots.
The present invention is based on the consideration that the mechanical driving point impedance is highest at the resonant frequency of the system. The latter impedance, as well as the Q of the system, determine the force necessary for tracking in the low frequency range. Although high stylus compliance will minimize the tracking force, particularly in the low frequency audio range, any increase of stylus compliance lowers the resonant frequency of the tone arm system of which the stylus is a part. Thus, there is an increase in the force necessary to track record warps and low frequency audio modulations. The effective mass of the system acts as a limit on total system performance since it cannot be reduced below the mass of the cartridge in a conventional system. This lower limit on the mass therefore sets a maximum compliance limit of the stylus.
Accordingly, in the present invention, a pivoted tone arm system is employed which includes one or more feedback loops energized by the amplified output signal of a tone arm-mounted cartridge. Each feedback loop includes a compensation network connected to corresponding torque motor which applies a variable force in a selected direction to the tone arm to modify the normal parameters of the tone arm system. With the resonance quality factor and the system resonant frequency selectively set at predetermined values, a high compliance stylus may be used which permits the tracking force to be kept low in the warp frequency range as well as in the low audio frequency range.
The foregoing and other objects of the present invention, together with the features and advantages thereof will become apparent from the following detailed specification, when considered in conjunction with the accompanying drawings.